1. Field of the Invention
The present invention relates to resonant power supply circuits, and more particularly, the present invention relates to a resonant power supply circuit for generating a high voltage to be supplied to a cathode ray tube (CRT) or other electronic apparatus.
2. Description of the Related Art
FIG. 8 shows an example of a resonant power supply circuit related to the present invention. A resonant power supply circuit 10 includes a flyback transformer 12. The drain of a field-effect transistor (FET) 14 functioning as a switching device is connected to a first end of a primary winding of the flyback transformer 12, which has a source that is grounded. Between the drain and the source of the FET 14, a resonant capacitor 16 and a damper diode 18 are connected in parallel. The anode of the damper diode 18 is connected to the source side of the FET 14, and the cathode is connected to the drain side of the FET 14. A second end of the primary winding of the flyback transformer 12 is connected to a power supply 20.
A secondary winding of the flyback transformer 12 is connected to a voltage divider circuit 24 through a diode 22. A signal having a voltage that is divided by the voltage divider circuit 24 is input to a control circuit 26, thus generating a control signal to be input to the gate of the FET 14.
FIG. 9 shows waveforms of signals at sections of the resonant power supply circuit 10. Specifically, trace (a) shows the waveform of a voltage at point A in FIG. 8; trace (b) shows the waveform of a current in the primary winding of the flyback transformer 12; and trace (c) shows the waveform of a signal for controlling the FET 14. When the FET 14 is turned ON at t0, current flows from the power supply 20 to the primary winding of the flyback transformer 12 and the FET 14. As a result of the current, electromagnetic energy is stored in the primary winding of the flyback transformer 12.
When the FET 14 is turned OFF at t1, current flows from the primary winding of the flyback transformer 12 to the resonant capacitor 16, thus causing the primary winding of the flyback transformer 12 and the resonant capacitor 16 to resonate with each other. As a result, as shown in trace (a) in FIG. 9, a flyback pulse is generated. The flyback pulse reaches a maximum value when the entire electromagnetic energy stored in the flyback transformer 12 is converted into electrostatic energy in the resonant capacitor 16.
When the flyback pulse reaches the maximum value, the electrostatic energy in the resonant capacitor 16 is inversely converted into electromagnetic energy in the primary winding of the flyback transformer 12, and the voltage of the flyback pulse decreases. When the flyback pulse becomes zero at t2, the damper diode 18 is turned ON, thus causing current to flow from the ground side to the primary winding of the flyback transformer 12. When the voltage at point A recovers to the voltage of the power supply 20, the damper diode 18 is turned OFF, and the current becomes zero. When the FET 14 is turned ON at t4, current flows from the power supply 20 to the primary winding of the flyback transformer 12, thus returning to the initial state at t0. The operation as described above is repeated to maintain the circuit operation. The flyback pulse is boosted by the flyback transformer 12, which results in outputting a high voltage from the secondary winding.
At t3 in which current becomes zero, resonance with the primary winding of the flyback transformer 12 occurs due to the capacity of the resonant capacitor 16 and parasitic capacity in the FET 14. Between t3 to t4, a ringing pulse is generated. Such a ringing pulse causes noise. In order to prevent the generation of a ringing pulse, as shown in FIG. 1 that illustrates another related art device, a clamping circuit 28 is provided. When a ringing pulse starts to be generated, the clamping circuit 28 causes both ends of the primary winding of the flyback transformer 12 to have the same voltage, thus preventing resonance.
The clamping circuit 28 is defined by a series circuit including a diode 30 and an FET 32 that functions as a second switching device. The clamping circuit 28 is connected in parallel to the primary winding of the flyback transformer 12. The operation of the FET 32 is controlled by the control circuit 26.
Referring to FIG. 10, a control method includes a method of simultaneously turning ON the first FET 14 and the second FET 32, the first FET 14 functioning as the first switching device for generating a flyback pulse and the second FET 32 functioning as the second switching device used in the clamping circuit 28, when the voltage at point A is zero. After the first FET 14 is turned OFF, the second FET 32 is turned OFF.
According to this method, the second FET 32 is turned ON to cause both ends of the primary winding of the flyback transformer 12 to have the same voltage. Even when the first FET 14 is turned ON and current flows through the primary winding, the voltage at point A remains at zero. When the first FET 14 is turned OFF and electromagnetic energy in the primary winding of the flyback transformer 12 is converted into electrostatic energy in the resonant capacitor 16, the voltage at point A becomes equal to the voltage of the power supply 20. When the second FET 32 is turned OFF, a flyback pulse is generated.
According to the control method as described above, the first FET 14 can be operated when the voltage at point A is zero, thus preventing the generation of a ringing pulse. When the first FET 14 is turned OFF, current flowing through the primary winding of the flyback transformer 12 reaches its maximum level. This current flows backward by a route passing through the diode 30 and the second FET 32. A circuit loss caused by the backflow is greater than that in a case in which no clamping circuit is provided.
FIG. 11 shows another control method. According to this method, the second FET 32 is turned ON when the voltage at point A is zero. In this state, when current in the primary winding of the flyback transformer 12 becomes zero, the voltage at point A becomes equal to that of the power supply 20 because the voltages at both ends of the primary winding are clamped. After the first FET 14 is turned ON, the second FET 32 is turned OFF.
According to the control method as described above, while backflow current as shown in FIG. 10 is eliminated, a large switching loss is caused since the first FET 14 is turned ON at the same time the voltage at point A becomes equal to that of the power supply 20. Although a voltage ripple is suppressed, current noise is generated. Such current noise in turn generates screen noise in a CRT and causes an increase in temperature of a flyback transformer. It is thus necessary to provide a damping circuit to remove the current noise. In such a case, a considerably great loss is caused in the damping circuit.
In order to solve the problems described above, preferred embodiments of the present invention provide a resonant power supply circuit that effectively minimizes a circuit loss and noise.
According to a preferred embodiment of the present invention, a resonant power supply circuit includes a flyback transformer, a power supply for supplying power to a primary winding of the flyback transformer, a first switching device for controlling current which flows from the power supply to the primary winding of the flyback transformer, a resonant capacitor for generating a flyback pulse by resonating with the primary winding of the flyback transformer when the first switching device is OFF, and a clamping circuit including a diode and a second switching device which is connected in parallel with the primary winding of the flyback transformer, whereby the voltage between both ends of the primary winding of the flyback transformer is clamped. When T represents a period of a ringing pulse which is generated after the second switching device is turned OFF, the first switching device is turned ON after a period ranging from about {(n+xc2xd)T} to about {(n+1)T} (where n is zero or a natural number) elapses from the time when the second switching device is turned OFF.
Preferably, the first switching device is turned ON after a period of approximately {(n+xc2xe)T} (where n is zero or a natural number) elapses from the time when the second switching device is turned OFF.
More preferably, the first switching device is turned ON after a period of approximately (3T/4) elapses from the time when the second switching device is turned OFF.
By turning ON the second switching device of the clamping circuit which is connected in parallel to the primary winding of the flyback transformer, the voltages at both ends of the primary winding are clamped, thus preventing the generation of a ringing pulse.
A ringing pulse is generated by turning ON the second switching device. If T represents a period of the ringing pulse, the voltage level of the ringing pulse is low in a period ranging from approximately {(n+xc2xd)T} to approximately {(n+1)T}. By turning ON the first switching device within this period, the generation of current oscillation is minimized.
In particular, the position of approximately {(n+xc2xe)T} is the lowest level of the ringing pulse. By turning ON the first switching device at this position, the generation of current oscillation is prevented.
By turning ON the first switching device at n=0, that is, after a period of approximately 3T/4 elapses from the time when the second switching device is turned OFF, the first switching device can be turned ON at the lowest level of a first ringing pulse. Since the ringing pulse does not continue for a long period of time, the ON-time of the first switching device can be increased. By adjusting the ON-time of the first switching device, a high output voltage of the flyback transformer can be adjusted.
According to preferred embodiments of the present invention, a resonant power supply circuit is controlled so that a first switching device is turned ON after a predetermined period of time disclosed above elapses from the time when a second switching device is turned OFF, thus minimizing circuit loss and noise generation. Since less noise is generated, a CRT screen is not affected. This eliminates the necessity for a damping circuit for suppressing noise.
Other features, elements, steps, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments with reference to the attached drawings.